Alexander disease (AxD) is caused by dominant point mutations in the gene encoding glial fibrillary acidic protein (GFAP), an intermediate filament expressed in astrocytes of the central nervous system (CNS). A major factor in the pathology of AxD is astrogliosis and the concomitant increase in GFAP expression that exacerbates protein accumulation and GFAP toxicity. However the direct cause of GFAP transactivation is not known. In Gfap+/R236H mouse models of AxD, reactive astrocytes with GFAP aggregates are present throughout forebrain regions including hippocampus, corpus callosum and olfactory bulb, while hindbrain regions show minor or no increase in GFAP. Our recent findings show that the transcription factor and signal transducer STAT3, well known as a regulator of gliogenesis, astrogliosis, and GFAP expression, is also activated in astrocytes in AxD. In the mouse models of AxD, STAT3 activation coincides with astrogliosis and increased GFAP in forebrain astrocytes, with little activation in hindbrain regions. In this proposal, to understand the role o STAT3 in astrogliosis and the heterogeneous response to mutant GFAP, we will employ our Gfap+/R236H mice to determine whether STAT3 activation is necessary to sustain chronic gliosis, and to identify astrocyte specific gene networks underlying the differential response to the same genetic insult. In Specific Aim 1, we will test whether STAT3 is part of a positive feedback loop that drives GFAP expression and astrogliosis in AxD. Both genetic and pharmacological inhibition of the JAK2/STAT3 pathway will be used to determine its contribution to astrocyte activation and increased GFAP expression. First, we will cross Gfap+/R236H and STAT3 knockout mice to analyze whether decreased STAT3 levels result in a corresponding decrease in GFAP. In a second set of experiments, the JAK2 inhibitor AZD1480 will be used to treat both young and adult Gfap+/R236H mice and determine whether blocking STAT3 activation can reduce gliosis and GFAP accumulation. Our AxD mouse models have been well characterized and further analysis of established phenotypes will show whether reducing STAT3 affects the stress response and other cell types in the CNS. In Specific Aim 2, we will focus on regional heterogeneity in STAT3 activation and astrogliosis. Translational profiling will be used to isolate astrocyte specific transcripts and identify the gene networks that differentially regulae gliosis in forebrain versus hindbrain astrocyte populations. In addition to identifying pathways regulating STAT3 activation, these experiments will define new mechanisms controlling astrocyte reactivity in different brain regions. Given the current interest in astrocytes as therapeutic targets in other proteinopathies and neurodegenerative disease, these studies will provide insight into how region specific pathology may impact therapeutic strategy. This will be the first study to identify the intrinsic differences in regional astrocyte populations that contro reactive gliosis in a genetic model of neurodegeneration.